1. Field of Invention
The invention relates to a method and device for galvanizing plate-shaped products in horizontal continuous plants, particularly for electrolytic treatment of printed circuit boards and printed circuit films.
2. Brief Description of the Prior Art
An arrangement of this type is described in the European Patent Specification 0254030 B1. The products are transported horizontally through the electrolytic plant. The anodes are located along the transport path in the form of plates or cages, which are filled with the metal to be deposited. During its passage through the electrolytic plant, the product is coated in a known way by this metal.
As it enters the electrolytic plant, the plate-shaped product is grasped at its edge by transport members in the form of clamps. The clamps release the product again at the end of the electrolysis chamber. The clamps, secured on an endless revolving transport means, are returned to the inlet to the electrolytic plant.
Apart from transport, the clamps serve also for cathodic contacting of the product during its horizontal passage through the electrolytic plant. For this purpose the plants comprise an electrically conductive material. Furthermore, each clamp is provided, outwith the electrolysis cell, i.e. above the bath level, with a wiper arrangement for transmitting the electrolytic current. Due to the cathodic polarity of the clamp, it is coated with a metal to be deposited within the electrolysis cell just like the product. In order to avoid continual coating during each revolution, the clamps are again de-metallized in a de-metallizing chamber during the return journey. For reasons of system technology, the de-metallizing chamber can only contain the electrolyte which is also located in the electrolysis cell. The electrolyte is optimised for metallization, but not for de-metallization. The massive deposition of metal is frequently not entirely removed by electrolytic de-metallization. In the manufacture of printed circuit boards, it also appears as a disadvantage that the clamps, which are highly electrically conductive and cathodically poled, act as a "thief cathode". The consequence is that the layer deposited on the printed circuit board in the vicinity of the clamps is insufficient. This undesired deposition of metal gives rise to additional consumption of anodes and of energy, particularly for de-metallization.
A further horizontal continuous plant for electroplating printed circuit boards is described in DE 42 12 567 A1. This system uses as a contacting means rods with grippers, which grasp the plates not at the side but at the front edge, drawing them through the plant. The rods are likewise cathodically poled. As a result they are electroplated just like the product. Here also de-metallization is carried out as the rods return to the initial point. The disadvantages named also apply to this contacting means.
In order to avoid the named disadvantages, according to prior art the clamps or rods are covered with an electrically insulating material. This coating covers at least the area of the clamps or rods which projects into the electrolysis cell. Only the area in actual contact with the product, e.g. a printed circuit board, remains without insulation. Materials known in electroplating installations, such as PTFE, are considered as insulating materials; other suitable materials are glass, ceramics and enamel.
It has now become apparent in practice that, in spite of the electrical insulation on the clamps or rods, metal is to some extent still partially deposited. The causes of this are faults in the insulating layer, damage to this layer during use and the necessarily bright metal contact. At these points the field densities are so concentrated that disproportionate metallization takes place. This then cannot be entirely removed in the following de-metallization process, even if attempts are made to operate at this point with a higher current density. When the current density is too high, the electrically conductive connection from the layer to be removed to the clamp or rod burns out, i.e. the undesired deposits on the insulating layer represent a metallic island which can no longer be removed electrolytically, because it only has a high-resistance connection with the metallic base member. This connection however causes a situation in which, upon each passage through the electrolysis cell, growth of the island is to be observed. Despite the insulating layer on the contacting means, and de-metallization after each metallization stage, this leads to a situation in which, due to massive metal deposits, the contacting means must be exchanged from time to time. As these intervals are a few weeks apart, in addition to high costs for working time and replacement material, this also means loss of production.
It occurs in practice that the electroplating plant is not permanently supplied with product. The plant is often set in operation and stopped again after the end of production. In these cases the endless transport of the contacting means must continue at least until the last board has left the system. Over certain distances or over certain periods of time, therefore, contacting means remain in the plant, but no product. The contacting means, which have an ideal cathodic contact, then electroplate particularly intensely at all metallic open points, due to the electrical peak effect. This frequently leads to burns on the deposited metal layers, particularly around the actual contact point, which in this case is not grasping any product. The burns contaminate the electrolyte with microfine metal particles. These particles may only be filtered out with great difficulty. Particles in the electrolyte form nodules in the electrolytic layer on the product.
A method is known from DE 30 39 681 C2 for avoiding burns or excessive deposits on the product and on the contacting means. With respect to the position of the product beneath or above the anodes, these latter are switched on or switched off by means of high-current switching apparatus, so that unacceptably high field line densities on the product are avoided in the case of larger gaps in the sequence of product boards. If there is no product in the vicinity of the anode, then it is switched off. The contacting means which chance to be present at that point are then not exposed to any field line concentration. This effective but technically expensive measure still has a disadvantage with respect to electroplating plants with clamp units: printed circuit boards for example pass through such horizontal systems at a spacing of about 20 millimeters. In practice, the board lengths are variable. Synchronization of the position of the revolving clamps with the position of the printed circuit boards which have passed into the system at optional points in time, is not possible without disadvantage to a homogeneous distribution of layer thickness of the deposited metal. This leads to a situation in which clamps sporadically grasp into such a gap between the workpieces. They are cathodically poled and are located in the area of influence of the switched-on anodes. Faults in the insulating layer on the clamps and the contacts are then particularly intensively electroplated, leading, despite all countermeasures, to the disadvantages described.
The cathode potential of the stationary sources of bath current is transmitted to the revolving clamps by means of wiper brushes. Along the horizontal electroplating plant and parallel to the path of the revolving clamps there is located an extended wiper rail. Each clamp has a wiper brush, which is also known as a wiper contact. The wiper brush produces the electrical connection between the stationary wiper rail and the revolving clamp. This contacting is not required at the turning points of the belt transporting the clamps. Therefore the wiper rail terminates before the turning point. For de-metallization, the clamps are contacted during their return journey via a second wiper rail. In this case the clamps receive an anode potential from a further source of bath current. This wiper rail terminates in front of the second turning point. Thereafter the clamps are again coupled to the described rail with cathode potential for metallization. During an entire revolution of a clamp, therefore, the wiper brush runs twice on to a wiper rail and down again. In this way the supply of electrical bath current is ensured. In addition each clamp must grasp the incoming product by closing the clamps after the turning point at the end of the inlet path. The clamp must again release the material before the start of the outlet turning point. These closing and opening movements are constrained by stationary inclined planes at the turning points, over which correspondingly shaped cams on the clamps are automatically guided. This means that closing and opening are at the same time caused by the drive system for the revolving clamps. A spring in the clamp provides the closing force itself. In known horizontal continuous systems, about 160 clamps at a respective spacing of 80 millimeters are attached on a revolving belt, e.g. on a toothed belt. The automatic guidance is independent of whether product is present or not in the vicinity of the clamps, i.e. whether clamp closure or electrical contacting are required or not. The disadvantages in the method result from this automatically guided arrangement.
The document GB-A-22 66 727 discloses a system for continuous electroplating of printed circuit boards. A product item is continuously electrically contacted in a uniform sequence. There is no provision for switching on and off the contacting means in a controlled manner in dependence on gaps in the product.
A dip bath electroplating plant is known from DE-B-25 12 762, in which product is secured to a revolving chain. At specific turning points in the area of the electrolytic bath, the workpiece carrier is contacted irrespective of whether product is located on the workpiece carrier or not. Upon its entry into the electrolytic bath, no workpiece is individually grasped, but is secured on the workpiece carrier (contacting means) and in this way passes through the entire system with a plurality of treatment stations, without being removed from the chain in the interim.
A device described in DE 39 39 256 C2 is known for protecting the contacting means against undesirable electroplating at the contact. If no printed circuit board is present in the contacting area, the contacting means is closed in such a way that the contact is fully electrically insulated with the aid of a seal member. In practice, printed circuit boards are of different lengths. The contacting means are not synchronized with the transport system for the printed circuit boards. Consequently, the contacting means also occasionally only partly grasp the edge of the printed circuit board. Thus the effect of the sealing element is removed. The contact electroplates at least partially. Furthermore, it occurs that the contacting means grasps the edge of a printed circuit board so ineffectively that the seal member and/or the insulating layers in the contact area are destroyed. The glass fiber reinforced printed circuit boards are extremely rough at their cut edges, and, together with the high contact force, they destroy the insulating means applied thereto.